1. Field of the Invention
The present invention relates to an electroconductive diffuse reflective film and a method of producing the same.
2. Description of the Related Art
Various types of reflectors are used in lighting apparatuses, decorative illumination signboards, displays, and the like. Examples of such reflectors include metal reflectors having a high visible-light reflectance and white reflectors that exhibit white reflection, and such reflectors are used in the form of a film, a sheet, or a plate.
Among these reflectors, metal reflectors include a reflective film made of a metal material such as silver, aluminum, or a stainless steel, each of which has a high visible-light reflectance. Such metal reflectors are used as a headlight reflector for an automobile, a fluorescent lamp reflector, an incandescent lamp reflector, a telescope reflecting mirror, a reflecting mirror for a solar cell, and a sunlight-condensing plate. Such metal reflectors are supplied as a film having a metal film or a metal film-coated plate. Physical vapor deposition such as vapor  deposition or sputtering is often used as a method of forming such a film having a metal film, or a metal film coating.
Furthermore, since such a metal reflector has a high reflectance and a high electrical conductivity, the metal reflector can be used as a reflecting electrode combining a function of reflecting light from the outside and a function of an electrode. For example, in some reflective liquid crystal displays, metal wiring for driving liquid crystal molecules and transistors also functions as a reflecting electrode. Examples of a product that requires a similar reflecting electrode include a reflector of a solar cell electrode, electronic paper, and an electrochromic display.
Most visible light incident on a surface of a metal reflector is totally reflected in a direction of an angle of reflection equal to an angle of incidence, and thus, the mirror reflectance, that is, the regular reflectance is high. When the angle of reflection deviates from the regular reflection, the reflectance significantly decreases. In addition, as is apparent from the fact that a metal plate functions as a mirror, the surroundings are reflected at the surface of the metal reflector as a mirror image. Therefore, a metal reflector may be disadvantageous when being used as a reflecting electrode. For example, a metal reflector is used in a reflective display device. In this case, an image  of a viewer may be reflected on a screen at some angles because of mirror-like reflection caused by the reflector. In addition, glare of the screen occurs at an angle at which reflection is strong. Furthermore, reflection is strong at certain angles and is weak at other angles. Accordingly, a viewer may recognize variations in the screen because of the nonuniform reflection. For such a reflective device, reflection similar to reflection that occurs at white paper, in which the mirror reflectance is decreased and the diffuse reflectance is increased, can provide uniform and smooth impressions rather than mirror reflection similar to reflection that occurs at a mirror at which reflection strongly occurs.
On the other hand, a white reflector is used as a reflector of a fluorescent lamp or an incandescent lamp on which reflection or glare does not occur. For example, a resin application method, a method of coating a white pigment, a method of sintering a ceramic powder such as a barium sulfate powder are practically used as a method of preparing such a white reflector. Since such a white reflector is composed of an insulator, the white reflector does not function as an electrode and cannot be used as a reflecting electrode. If a white reflector has a high electrical conductivity, characteristics of images can be improved. 
Furthermore, insulating white reflectors are used as reflectors in various fields. For example, in a transmissive liquid crystal display, an insulating white reflector is used as a backlight reflector. In general, a fluorescent tube, an electroluminescent (EL) plate, or a LED light is used as a backlight disposed on the back surface of such a transmissive liquid crystal display. A reflector for efficiently utilizing light from the backlight is used at the back surface of the backlight. A metal mirror plate has been used as this reflector, but the use of a white reflector has also been proposed. For example, Japanese Unexamined Patent Application Publication No. 2006-335935 proposes a thermoplastic resin foam having fine air bubbles inside as a reflector suitable for use in a light reflector of a display.
In addition, recently, as the output of backlights (fluorescent tubes) of transmissive liquid crystal displays has increased, the temperature of backlight units has also increased. To solve this problem, it has been necessary to take heat-dissipation measures. An example of such a heat-dissipation measure is improvement of the heat-dissipating effect. For example, Japanese Unexamined Patent Application Publication Nos. 2006-258849 and 2006-123463 propose that a heat-dissipating film be formed on a white pigment film provided on a surface of an aluminum plate. Furthermore, as  another example of a heat-dissipation measure, it is also effective to improve thermal conductivity of a reflector. In general, in metal materials, heat conduction due to free electrons is dominant, and thus a material having a high electrical conductivity also has a high thermal conductivity. Accordingly, if a metal material having a high electrical conductivity can be used as a white reflector, such a white reflector is also effective from the standpoint of a heat-dissipation measure because of its high thermal conductivity.
In a method of applying an insulating white coating material onto a metal base material, the effect of heat conduction is significantly decreased by applying the insulating material on a surface of the metal base material, though a metal plate in used as the metal base material. Therefore, it is preferable that such a coating is not used. Furthermore, electroconductive white reflectors can be used not only for display but also for illumination, decorative illumination, a solar cell electrode having irregularities thereon, and the like. A material that achieves white reflection and that has a high electrical conductivity has been desired in these fields.
However, known metal reflectors have a high electrical conductivity, but achieves low diffuse reflection. On the other hand, white reflectors on which painting, a resin coating, or an oxide powder coating is performed can achieve  high diffuse reflection, but do not function as an electrode because such white reflectors are made of an insulator. Accordingly, it has been difficult to obtain a reflective film and a reflector that can combine white diffuse reflection and electrical conductivity (a low electrical resistivity).